The invention relates generally to processes involving the cultivation of microorganisms. Of particular interest to the invention are processes for the cultivation of nutrient unicellular proteins.
Microorganisms such as yeats, bacteria and fungi are today being cultivated in large quantities so as to produce protein-rich substances. The proteins obtained from microorganisms of this type are unicellular proteins and are given the designation "single cell proteins" (SCP).
The culture medium for the cultivation of the microorganisms requires a source of carbon. For this purpose, it is known to use carbohydrates, paraffins or paraffin hydrocarbons, alcohols and other substances which are rich in carbon.
There are many problems which must be taken into account for such cultivations. For instance, a number of other substances such as phosphates or phosphorus, nitrogen, potassium, calcium, magnesium and trace elements which participate in the synthesis of the cells must be added to the carbon source. Similarly, certain organisms require the addition of working substances such as biotin and yeast extract among others.
It is known that, subsequent to recovery of the microorganisms from the culture medium, a portion of the waste water or used culture medium may be recycled for use in the fermentation or cultivation. However, it has been found that this recirculation can be carried out to a certain point only. The reason is that, aside from the unused salts and working substances present in the used culture medium, the used culture medium also contains proteins, and particularly enzymes, which have been formed therein and which can no longer be utilized by the single cells. In fact, the proteins which have been formed in the used culture medium may even have a growth-inhibiting effect. Accordingly, recirculation of the used culture medium with all of its associated components for use in the cultivation cannot be carried out beyond a certain point.
Since recirculation of the used cultured medium is possible to a certain point only, a portion of the used culture medium must be discharged as waste. However, the quantities of the above-indicated substances present in the used culture medium pose great problems as regards water pollution. In particular, it has been found that the biochemical oxygen demand (BOD), which is a measure of the polluting ability, may reach values of several thousand milligrams per liter.